1. Field of the Invention
The present invention relates to a voltage resonant inverter, a control method, and a surface treatment device.
2. Description of the Related Art
Inverters are used for supplying high voltage to various kinds of devices such as a discharge tube for a large-scale plasma display and a plasma generator. Inverters with output power of several watts are widely used, and inverters with alternating output having an output voltage of several kilovolts and electrical power of over several tens of watts are particularly used for plasma generators and other devices.
Examples of a power source device for supplying such kinds of devices with a high voltage include a high-voltage inverter described in Japanese Laid-open Patent Publication No. 2012-186984. As illustrated in FIG. 18, this kind of inverter has a step-up transformer T including a plurality of transformers T1 and T2 each having an independent core and a common characteristic. Such an inverter excites the step-up transformer T by switching an input voltage Vin with a switching element Q on which a control circuit performs on/off controls and simultaneously applying an excitation current to excitation windings Np1 and Np2, which are connected in parallel, of the respective transformers T1 and T2.
Output windings Ns1 and Ns2 of the respective transformers T1 and T2 are connected in series, and an output voltage Vout in an alternating high voltage where voltage waveforms induced by the output windings Ns1 and Ns2 are superimposed is output to a load. For example, a load such as an electrode of a plasma generator having load capacity Co is connected. Japanese Laid-open Patent Publication No. 2013-31338 describes such a technique that performs a pulse width modulation (PWM) control on a switching signal so as to make peak voltage values constant in an inverter with an alternating output having a peak voltage value of over ten kilovolts.
However, such an inverter outputting an alternating high voltage is a voltage resonant inverter in which voltage resonance is caused by inductance generated by an output winding and composite capacitance of stray capacitance or parasitic capacitance between windings and load capacitance in the secondary side of the step-up transformer. The voltage resonance generates a voltage in high-order attenuation waveform subsequent to a fundamental wave in a switching frequency, which accordingly generates a period (a time under an inhibiting condition) during which a switching element cannot be turned on. This configuration problematically limits a period for performing the PWM control, which accordingly prevents the inverter from performing a sufficient control to make peak values of the output voltage constant.
In view of the above, there is a need to considerably reduce time (a time under an inhibiting condition) in which a switching element cannot be turned on and extend control range for making peak values on the output voltage constant in a voltage resonant inverter as described above.